Is it (ever) rational to believe that the earth is flat?

To begin with, let me get you up to speed on the conversation.

In the thread to my last post I wrote this:

Consider Billy, raised on a compound of the Flat Earth Society in the Nevada countryside and taught to believe that the earth is flat (with all the standard lines of "evidence"). Is ten year old Billy justified in his belief? Of course. Will he be forever justified? If he stays on the compound then maybe. But if he pursues a degree in astronomy at UNLV he will very quickly encounter defeaters to his belief which will remove the justification.

Gaga wrote in reply "I find the (unintended?) irony of comparing yourself with a kid in a backwater compound amusing..." More seriously, Gaga then added:

yes I'm getting frustrated, because with every comment your po-moesque stance shine through a little more and that makes it difficult to argue with you.
You pull out this trick of the plausibility framework every single time someone tries to tease some detail out of your ideas. You might find it a powerful rethorical tool, but for me, I'm sorry, that's a cop-out.

I am not sure what Gaga means by "po-moesque". It is obviously a reference to postmodernism, but that is a huge umbrella term. Does he mean to suggest that I hold a form of epistemic relativism where literally anything goes? Surely not (I hope) since I have argued strongly for the place of defeaters in the rational defense of belief.

Gaga's dismissal of young Billy's justification for believing the earth is flat disturbs me, and so this is the point on which I'll focus. Would it be rational for Billy to believe the earth is flat if he lived in a Vandal tribe in fifth century Europe? Assuming that this is what the tribal chieftain Olavaar and the wise man Clovis,and Billy's own dad Eric Bloodaxe all believed and taught, then surely it would be rational for him to believe it too. (Need it be added that it certainly jibes with prima facie experience of the world?)

So it is possible for Vandal Billy to be justified in believing the earth is flat, even while I am not so justified. 

But what if Billy was raised in a compound in rural Nevada? Essentially he could be in a doxastic (belief-forming) environment very much like that of Vandal Billy. And so if it is rational for vandal Billy to believe this, then it is rational for Nevada Billy to believe it as well.

I don't find any postmodern skepticism in these claims. Rather, I find a common sense observation that, while certain principles of justification may be universal, people nonetheless have different starting points and evidence bases, and this leads to different justification outcomes. 

What we need to do, as I have repeatedly said, is talk to each other and do our level best to communicate the grounds for our belief that are appreciable by the other person. And together we can all move toward truth.